1. Field of the Invention
The invention relates to a control system for a vehicular automatic transmission and, more particularly, to a technique for controlling the automatic transmission so as to prevent the roll back (or retreating or reverse) of the vehicle at the start in a high speed stage on a snow-covered road and to effect the hill hold under a neutral control.
2. Related Art
When the vehicle is to be started on a low-.mu. road such as a snow-covered road by shifting the transmission to the lowest speed stage (i.e., the first speed), the drive force may become excessively high in relationship to the frictional force between the wheels and the road surface, thus causing wheel spin and making a smooth vehicle start impossible. A conventional automatic transmission is known that employs a technique enabling the vehicle to be started at a higher speed stage, such as in second or higher speed, in which the torque amplification in the transmission is smaller than that at the lowest speed stage, for starting the vehicle on a snow-covered road, as disclosed in U.S. Pat. No. 4,941,096.
In order to reduce the fuel cost during the running time of the vehicle, when the vehicle is stopped with the running range of the automatic transmission being selected, the engine load is lowered by causing the input clutch of a speed change mechanism, as ordinarily kept in the applied state, to stand by in the released state just before being applied, to reduce the dragging of the fluid in the torque converter. This is called the "neutral control". In order to prevent the vehicle from rolling back on a slope, as might otherwise occur because the creep force is eliminated by the neutral control, there is a technique which performs the hill hold by locking the speed change mechanism, as disclosed in U.S. Pat. No. 4,648,289.
When the control of the start at a high speed stage, as disclosed in U.S. Pat. No. 4,941,096, is made, the creep force may be small for the start on the low .mu. road such as the snow-covered road in relation to the slope gradient, and the vehicle may roll back simultaneously as the brake pedal is released. In this case, the drive force may become excessive and cause slippage of the wheels, thereby making a smooth start impossible if the driver depresses the accelerator pedal too quickly. A device could be provided for fixing the output shaft of the transmission, for example. However, it would be necessary to control the device such that the device is actuated only when the vehicle may possibly roll back and is quickly released simultaneously with the start. This raises a problem that the timing control is difficult.
In the hill hold control during the neutral control, as disclosed in U.S. Pat. No. 4,648,289, a frictional engagement element to be applied at the time of establishing the second speed of the speed change mechanism is used and applied in the neutral state. As a result, the speed change mechanism assumes the second speed configuration at the hill hold time so that a shift shock from the second speed to the first speed occurs when the release of the frictional engagement element is delayed. The problem of this case is that the timing control is also difficult.